Many imaging processes utilize interactions between leuco dyes and photolytically or thermally generated oxidants. Critical characteristics for the success of these leuco dyes and oxidants include their stability and handlability during storage prior to and subsequent to imaging as well as the imaging speed of such. Often, however, increased stability results in reduced imaging speed.
The ease of oxidation of these leuco dyes often determines their imaging speed, i.e., the amount of color formed based on the applied radiation. Thus, the more easily oxidized leuco dyes form color more readily and as such can be imaged with less energy. On the other hand, the more ambiently stable leuco dyes, e.g., tris(p-diethylamino-o-tolyl)methane can be more easily handled in room light, but require considerably more energy to produce effective image color when exposed to UV-light, heat or near-IR energy when formulated in comparable systems. A balance thus exists between the ease of oxidizing the leuco dyes, with concomitant lower roomlight stability, and the use of more roomlight-stable leuco dyes which yield slower imaging systems.
Imaging systems described in the literature often contain compositions including halocarbons or halocarbonyl compounds and leuco dye(s). Compositions including hexaarylbiimidazole (HABI) compound(s) and leuco dye(s) that are imaged using ultraviolet radiation (UV means) are also known. Also available are leuco dye lactones and developers, e.g., bisphenol-A or other phenolic derivatives.
A common type of leuco dye is Leuco Crystal Violet and its analogs. The literature contains descriptions of many additives which are reported to improve the stability of these leuco dyes. Among these are phenols, hydroquinones, phenidone, hindered amines, etc.
In many systems, the use of UV-absorbing agents has been reported to enhance the handlability of the unimaged and imaged coatings. However, in many instances, the use of light filtering materials has been found to be only partially effective.
In order to form useful images, it is necessary to stabilize the imaging system, i.e., prevent color to form in the background. Systems have been described which generate an inhibitor to color-formation by exposure to light (for example, U.S. Pat. No. 3,390,996), heat (for example, U.S. Pat. No. 3,390,995) or light and heat (for example, U.S. Pat. No. 3,383,212, and U.S. Pat. No. 4,332,884).
Modifications in which encapsulation is employed to control background and/or image stability have been reported. Thus a number of patents (for example, U.S. Pat. Nos. 4,929,530, 4,962,009 and 4,981,769) describe systems in which color-formation is effected inside capsules and heat is employed to rupture these in order to make contact between the components of color formation with chemical fixing agents outside the capsules so as to stabilize the imaged areas. Here the capsules are involved in separating the imaging and fixing components and to provide a relatively stable system. All these systems require some modicum of image stabilization to prevent significant color buildup in the unimaged areas, and "add-on" of images to previously exposed image areas is difficult. There is a need for a system that does not undergo significant changes when viewed in ambient light, thus reducing the ability to form color effectively when re-exposed.
In other cases, color formation was prevented by restricting the mobility of color formation, either by use of a thermoplastic binder, which required heating of the composition before, during or subsequent to UV exposure to allow color to form (for example, U.S. Pat. No. 3,615,481). A system also described stabilization of background after color-formation as a result of a subsequent polymerization step, effected by light or heat (for example, U.S. Pat. No. 3,615,454) which locked the color-forming components in place.
The thermally imageable compositions that are known use high levels of phenolic compounds with relatively reactive leuco dyes for improved stability. It has been found that high levels of the phenolic compounds do not give the required level of room light stability. The leuco dyes such as Leuco Crystal Violet which provide a more intense, visually more attractive color are limited in their use because of the ease with which these may be air oxidized. Also, the high levels of stabilizer required increase cost.
Hence, there has been a search for stabilizers which allow the formulation to include the more readily oxidized leuco dyes. Specifically, the search often focused on those which involve the stabilization of the leuco dye itself, rather than those which interact with one of the image-generating oxidants. However, a need still exists for a non-photosensitive, thermally sensitive compositions that exhibit good imaging speed in addition to having good room light stability.